1,701 research outputs found
Irrigation Decisions Simplified with Electronics and Soil Water Sensors
Two simple, inexpensive systems use electrical resistance measurements
to provide useful, immediate information to assist decisions
made on irrigation water application. in one system a microprocessor-based
circuit coupled to a programmable calculator provides
an on-site estimate of the time until the next irrigation will be required,
based on field data and an operator-supplied parameter. The
second system simply signals the arrival of the wetting front at any
location in the soil by giving a visual indication, such as raising a
mechanical flag. The microprocessor-based circuit measures and
stores the resistance of four gypsum blocks once a day. The program
in the portable calculator accesses this information and uses it to
extrapolate the soil drying rate to predict the number of days until
the next irrigation. By restricting the microprocessor circuit to data
acquisition only and putting all number-handling routines into the
calculator program, the cost and complexity of the microprocessor
circuit is minimized, whereas maximizing the programming flexibility.
This makes it feasible to install a number of these devices at
different locations, all serviced by the same portable calculator.
The water infiltration circuit intermittently scans eight sets of
stainless steel electrodes to locate the soil wetting front during irrigation.
When the resistance across the electrodes decreases, sig-
'sling the arrival of the front, the circuit trips a spring-loaded Bag.
This provides a visible sign that the wetting front has reached that
point in the soil. The equipment worked well. When irrigation was
required in six or fewer days, the microprocessor/calculator system
made correct predictions 85% of the time. An example of how easily
any irrigation scheduling method may be converted to the microprocessor/calculator
system is presented
Accumulation of Mercury Vapor in Soils Laboratories
The rate of accumulation of mercury vapor in a closed room
was determined as a function of surface area of the exposed
mercury, ambient temperature, and air flow. The rate of mercury
vapor accumulation was much higher than suspected by
many soil scientists. For example, in a 3- by 3- by 2-m constant
temperature room, 250 cm² of mercury surface produced toxic
levels of mercury vapor within 13 minutes. Water or oil covering
the surface of the mercury was a highly effective means of
suppressing vaporization. The standard decontamination treatment
with sulfur proved effective, but only after complete
amalgamation occurred
Plant Moisture Stress: A Portable Freezing-Point Meter Compared with the Psychrometer
A small portable instrument for measuring the freezing-point
depression of plant tissue has been developed for
field use. The instrument is easy to operate and can be
constructed from materials costing less than $100.
Moisture stress measurements made with the freezing-point
meter on a variety of plants were compared with
vapor pressure psychrometer measurments. Variation between
duplicates in the freezing point averaged 1.2 bars,
but differences between stress measurements made with
the psychrometer and freezing-point instrument averaged
2.6 bars
Plant Water Potential Gradients Measured in the Field by Freezing Point
A portable freezing point meter was used in the field to
measure the water potential gradients in sunflower (Helianthus
annuus), beans (Phaseolus vulgaris), corn (Zea mays),
wheat (Tritium aestivum), pumpkin (Cucurbita pepo), potato
(Solanam tuberosum), alfalfa (Medicago sativa), and sugarbeets
(Beta vulgaris). The measurements were made between
daybreak and sunrise, and again during the middle of the
afternoon on days when the potential evapotranspiration
varied between 6.5 and 8.0 mm of water.
The gradients varied from a maximum of 0.2 bar per cm
in a wheat, down to an undetectable value for pumpkin. Although
most of the soil in the root zone was kept at potentials
above –1 bar, the bulk of the root tissue had water
potentials of –5 to –10 bars. Differences in water potential
between shaded and unshaded leaves, and between leaf tissue
and guttation fluid suggested a similar drop of several bars
between xylem elements and the surrounding leaf tissue in
some plant species. The implications of such drops are discussed
with respect to plant water transport equations and
pressure cell potential measurements
Further study of the Over-Barrier Model to compute charge exchange processes
In this paper we study theoretically the process of electron capture between
one-optical-electron atoms (e.g. hydrogenlike or alkali atoms) and ions at
low-to-medium impact velocities () working on a modification
of an already developed classical In this work we present an improvement over
the Over Barrier Model (OBM) described in a recent paper [F. Sattin, Phys. Rev.
A {\bf 62}, 042711 (2000)]. We show that: i) one of the two free parameters
there introduced actually comes out consistently from the starting assumptions
underlying the model; ii) the modified model thus obtained is as much accurate
as the former one. Furthermore, we show that OBMs are able to accurately
predict some recent results of state selective electron capture, at odds with
what previously supposed.Comment: RevTeX, 7 pages, 4 eps figures. To appear in Physical Review A
(2001-september issue
Physical State of Water in Plant Xylem Vessels
The vapor pressure psychrometer was used as a tool
to study the physical state of water in plant xylem vessels.
The experimental procedure involved measuring
the change in diffusion pressure deficit (DPD) of corn
and tomato plants when the stem was cut. When the
DPD was greater than 4 bars in tomatoes and 28 bars
in corn, the water in xylem vessels no longer appeared
to flow in response to hydrostatic pressure gradients. The
limiting value of DPD increased as the xylem radius
decreased. A mechanism is suggested which describes the
physical state and the movement of water through xylem
tissue under high DPD. The proposal is based on the
pressure difference across a curved air-water interface
and on the concept of an electrostatic double layer with
its associated osmotic pressure
Steady and Stable: Numerical Investigations of Nonlinear Partial Differential Equations
Excerpt: Mathematics is a language which can describe patterns in everyday life as well as abstract concepts existing only in our minds. Patterns exist in data, functions, and sets constructed around a common theme, but the most tangible patterns are visual. Visual demonstrations can help undergraduate students connect to abstract concepts in advanced mathematical courses. The study of partial differential equations, in particular, benefits from numerical analysis and simulation
Properties of spin-triplet, even-parity superconductors
The physical consequences of the spin-triplet, even-parity pairing that has
been predicted to exist in disordered two-dimensional electron systems are
considered in detail. We show that the presence of an attractive interaction in
the particle-particle spin-triplet channel leads to an instability of the
normal metal that competes with the localizing effects of the disorder. The
instability is characterized by a diverging length scale, and has all of the
characteristics of a continuous phase transition. The transition and the
properties of the ordered phase are studied in mean-field theory, and by taking
into account Gaussian fluctuations. We find that the ordered phase is indeed a
superconductor with an ordinary Meissner effect and a free energy that is lower
than that of the normal metal. Various technical points that have given rise to
confusion in connection with this and other manifestations of odd-gap
superconductivity are also discussed.Comment: 15 pp., REVTeX, psfig, 2 ps figs, final version as publishe
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